Inkjet technology, which is well known for use in printing images onto paper, has also been employed in the fabrication of printed circuits. More particularly, inkjet printheads utilizing either thermal inkjet (TIJ) or piezoelectric (PZT) transducers have been employed to directly print circuit components onto circuit substrates.
There are two common manners in which fluid may be jetted from the printheads. One manner is known as drop-on-demand (DOD) and the other manner is known as continuous inkjet (CIJ). As the name suggests, DOD systems include systems designed to eject fluid through application of an electrical signal through a PZT element or a thin film resistor element when a drop is desired. CIJ systems, on the other hand, include systems designed to jet a stream of fluid that breaks up into droplets due to Rayleigh instability. In comparing these types of systems, DOD print heads have more complex fluidic design concerns but do not require a recirculation system for the ink. The DOD systems typically have much lower drop ejection frequency, drop velocity and larger drop size than CIJ systems because the fluid flow direction reverses during drop formation and break-off.
TIJ type printheads typically include a thin-film resistor for applying heat to superheat a small portion of the printing material above the heater element. During super-heating, a bubble of vaporized material is created and the explosive growth of the bubble expels the fluid in the nozzle of the printhead beyond the orifice to form droplets. The thin-film resistor is therefore positioned near and behind the nozzle and the printing material is fed to a location between the thin-film resistor and the nozzle opening. The use of TIJ type printheads is substantially limited by the types of materials that may be ejected from the printheads. For instance, TIJ type printheads may be unable to supply sufficient heat power to super-heat high melting temperature, thermally conductive materials.
PZT, or lead-zirconate-titanate, type printheads use PZT as a transducer whose dimensions change when it receives a voltage pulse. This dimensional change generates a pressure wave (acoustic wave) that propels the printing material through a nozzle of the printhead. PZT type printheads are not suitable for use in the typically high temperature environments necessary for application of many metals, inorganic semiconductors and ceramics because PZT elements commonly used as the actuators are limited by the depolarization temperature for their piezoelectricity.
Another drawback associated with DOD type printheads in printing circuit components is that its drop frequency is limited by the ac impedance associated with its fluidic circuit that employs ac type of fluid motion.
Therefore, it would be desirable to have a relatively simple and inexpensive printing device capable of printing a large number of various materials onto various types of substrates.